Recent years have seen rapid growth in the demand for inexpensive, lightweight and robust measuring and control devices. This demand has been manifested in the rapid developments in the field of miniaturized devices, such as lab-on-a-chip.
Display backlighting significantly contributes to the battery consumption of mobile devices, such as notebook computers, PDAs and mobile phones. Consequently, it is possible to considerably increase the useful lifetime of a battery by controlling the backlights so that they are dimmed in dark conditions and are only increased when there are high ambient light levels.
Beyond the above specific example, the provision of inexpensive, miniaturized light monitoring sensors and control systems (e.g., for car headlights, large building lighting networks, street lighting networks, etc.) will clearly provide significant economic and environmental benefits. Traditional light sensors typically produce an analog output signal. One of the main challenges encountered in previous attempts to provide integrated miniaturized light sensing and control systems has been the problem of combining analog signal processing circuitry with digital signal processing circuitry on the same chip.
Accordingly, prior art on-chip light sensors have typically possessed limited data processing capabilities. This has created particular problems since it means that such devices have limited, if any, ability to compensate for manufacturing variations between components.
For instance, the Microsemi LX1970 and 71 (http://www.microsemi.com/micnotes/1403. pdf) device is an 8-pin dumb-sensor that requires continual monitoring. Similarly, the TDK BCS series requires continual monitoring. The Texas Instruments TAOS (TSL230R/A/B, TSL235R, TSL245) devices possess a narrow dynamic range with no fine control on the limit and no matching or compensation for component errors.
Since the present invention relates to imaging sensors, and more particularly, to a light to frequency converter, it is useful at this point to briefly review the properties of CMOS image sensors and the operation of the light to frequency converter circuit.
A brief overview of CMOS image sensors will now be discussed. Recent advances in the design and fabrication of complementary metal oxide semiconductor (CMOS) chips have meant that CMOS imaging sensors are adopting a more dominant position in the low-cost imaging market.
One of the main advantages of CMOS imaging sensors is that they can be produced using standard fabrication procedures which are already widely used for producing CMOS chips for computer processors, memory chips, etc. Furthermore, the signal processing and control circuitry for a CMOS imaging sensor can be integrated directly onto the CMOS chip.
A light to frequency converter circuit will now be discussed. As an overview, a light to frequency (LTF) converter, as described in U.S. Pat. No. 5,850,195 discloses a CMOS imaging sensor with a large dynamic range. The LTF converter architecture possesses several advantages over traditional imaging sensors. These advantages primarily reside in the following features: integration capacitance tolerance, integration capacitance size, and frequency output. These features will be discussed in more detail below.
With respect to integration capacitance tolerance in a conventional light sensor, a photodiode's capacitance is defined by its well capacitance. However, this feature can be hard to control. Consequently, it is difficult to produce an array of photodiodes with matched sensitivity.
In contrast, an LTF converter employs a charge amplifier structure, which ensures that the effective capacitance of the LTF converter is determined by an integration capacitance provided by a feedback capacitor. Since capacitors can be manufactured with tighter controls over their capacitance (e.g., poly-poly or metal-metal capacitors), the variability in the capacitance of the individual LTF converters in an LTF converter array is less than that of a similar number of traditional light sensors.
With respect to integration capacitance size, increasing the size of a photodiode should in principle increase its ability to collect incident photons. As a result, this increases its sensitivity to incident light. Larger photodiodes also possess an increased parasitic capacitance. This has the effect of negating the ability of the photodiode to collect more photons, and thereby eliminates any sensitivity benefits of the increased photodiode size.
In contrast, the LTF converter employs a charge amplifier structure that isolates the capacitance of the LTF converter's photodiode from the rest of the LTF converter circuitry. This ensures that the effective capacitance of the LTF converter is determined by the capacitance of its feedback capacitor (as described above). Consequently, it is possible to use a large photodiode in an LTF converter while retaining a small overall circuit capacitance, and thereby producing a high sensitivity detector.
With respect to frequency output, on-chip signal processing with traditional analog light sensors is relatively sensitive to noise from the other on-chip circuitry. In contrast, the charge amplifier structure of an LTF converter is readily combined with a comparator to produce a digital signal whose frequency is proportional to the light on the LTF converter's photodiode.
The digital signal produced by an LTF converter is both robust and measurable over a large dynamic range (i.e., 140 dB of dynamic range is practical with the charge amplifier architecture). In addition, the LTF converter system is auto-exposing, insofar as no external control loop is required to ensure that an LTF converter's photodiode pixel does not saturate.
The operation of an LTF converter will now be described below with reference to FIGS. 1-5. The LTF converter comprises a control circuit 4, a photodiode 6 and a current to digital signal converter 8. The current to digital signal converter 8 uses a switched-capacitor charge metering technique to convert a photo-current from the photodiode 6 to a digital signal of a specific frequency. The current to digital signal converter 8 comprises a bias circuit 10 (which controls the maximum operating speed of the digital signal converter 8), an amplifier circuit 14, a switched feedback capacitor 16 in a charge sensing amplifier circuit (not shown), a comparator 18 and a monostable multivibrator circuit 19.
Referring to FIG. 2, the charge sensing amplifier circuit 20 effectively isolates the remaining circuitry of the current to digital signal converter (not shown) from the large capacitance of the photodiode 6 (<100 pF). The charge sensing amplifier 20 comprises an operational amplifier 22 configured in a closed loop configuration with its non-inverting input coupled to a reference voltage (Vrt) and its inverting input connected to the feedback capacitor 16. The reference voltage (Vrt) is set as low as possible to increase voltage swing while maintaining the depletion region of the PN junction of the LTF converter. The reference voltage (Vrt) is usually set to approximately 0.7V.
Since the operational amplifier 22 has a large input impedance, virtually no current flows through it. Consequently, the output of the operational amplifier 22 changes to ensure that the inverting and non-inverting inputs of the operational amplifier 22 remain at the same potential (i.e., Vrt) In the process, a current flows through the feedback capacitor 16 which has the same magnitude (but opposite sign) to the photo-current generated by the photodiode 6 (Ipd)
Equation (1) below shows the relationship between the output voltage (Vout1) from the charge sensing amplifier 20 and the photo-current generated by the photodiode 6.
                              V          out1                =                              -                          I              pd                                ⁢                                    T              int                                      C              fb                                                          (        1        )            
From the above expression it can be seen that the output voltage (Vout1) from the charge sensing amplifier 20 is independent of the photodiode's 6 capacitance. Referring to FIG. 3, the output voltage (Vout1) from the charge sensing amplifier 20 is accumulated until it reaches a maximum value (Voutmax) at which point it is reset.
FIG. 4 shows a system for resetting an integrating amplifier (not shown) in the amplifier circuit 14. In this system, the output voltage from the amplifier circuit 14 (Vout2) is transmitted to the comparator 18. In the comparator 18, the output voltage (Vout2) is compared against a reference voltage (Vref). If the output voltage (Vout2) exceeds the reference voltage (Vref), the comparator 18 transmits a control signal (CTRL) to the monostable multivibrator circuit 19. In response to the control signal (CTRL), the monostable multivibrator circuit 19 emits a pulsed signal (RESET) to discharge the feedback capacitor 16.
Consequently, the frequency of the control signal (CTRL) is also proportional to the photodiode current Ipd (assuming that the integrating amplifier in the amplifier circuit 14 settles completely during the period of the control signal (CTRL)). The control signal (CTRL) is also fed to a divide-by-two circuit 30 to form the overall output signal (Fout) from the LTF converter. By employing a divide by two circuit 30, a symmetrical output signal is produced, which is more reliably detected since it no longer includes short pulses.
Returning to equation (1), since the rate of change (slope) of the output voltage (Vout1) from the charge sensing amplifier is proportional to the intensity of the incident light, the frequency of the overall output signal (Fout) from the LTF converter is also proportional to the incident light intensity. This proportionality is more clearly expressed in equation (2) below.
                              F          out                =                              I            pd                                2            ⁢                                          C                fb                            ⁡                              (                                                      V                    ref                                    -                                      V                    rt                                                  )                                                                        (        2        )            
It can be seen from equation (2) that although the overall output signal (Fout) from the LTF converter is proportional to the photocurrent (Ipd) from the photodiode, it is also dependent on the reference voltages (Vref, Vrt) and the capacitance of the feedback capacitor Cfb. While it is possible to use bandgap reference voltages to accurately produce the above reference voltages, the capacitance of the feedback capacitor is less easily controlled since it is typically subjected to manufacturing variations.